The present invention relates to cigarette filters comprising activated carbon fibers, and more particularly to cigarette filters comprising a bundle of activated carbon fibers with or without particulate adsorbent incorporated therein for removing gas phase constituents from mainstream tobacco smoke through adsorption of such gas phase constituents by the activated carbon fibers.
Activated carbon filters for adsorption and separation have been used in cigarette filter constructions. When granular activated carbon is used in a plug-space-plug filter configuration, for example, great care must be taken to ensure the carbon packed bed leaves no open space for the smoke to by-pass the activated carbon bed. Open spaces such as channels in the carbon bed lead to filtration inefficiencies.
Activated carbon in granular form has been used in the past to remove gas phase constituents in the cigarette smoke. In such methods, the mainstream smoke is contacted with the bed of granular activated carbon to adsorb the constituents to be removed. The removal efficiency of such methods is typically limited by the adsorbing capacity of the adsorbent bed, which is dictated by the total surface area and volume of pores in the micropore region accessible to the smokestream. Conventionally, micropores are defined as pores with widths less than 20 angstroms. The removal efficiency by such methods is also limited by the above described phenomenon of by-passing through the granular bed, whereby the smokestream passes through the bed without sufficient contact with the adsorbent for effective mass transfer. To counteract the loss of efficiency resulting from the limitation of the latter type, a typical solution is to construct the filter with a superfluous and redundant amount of adsorbent material to compensate for the loss of efficiency through by-passing. Activated carbon beds of the loose granular type incorporated within a cavity in the cigarette filter are susceptible to by-passing because a 100% fill is required to ensure a “fixed bed” of adsorbent with minimized channels. Such 100% fill is rarely achieved on a uniform basis using high speed manufacturing machinery. Another typical solution to avoiding by-passing of smoke through the bed is to use particulates with small diameters to ensure intimate contact of adsorbate with adsorbent; however, this solution typically leads to undesirably high pressure drops across the filter.
Adsorbing materials such as activated carbons, zeolites, silica gels and 3-aminopropylsilyl substituted silica gels (APS silca gels) are porous materials capable of removing gaseous components from cigarette smoke. Most of the commercially available adsorbing materials are in granular or powder forms. Materials in granular forms have difficulty in achieving the design or performance in a cigarette filter due to settling after the manufacturing process, whereas materials in powdered forms create too high a pressure drop to be practical.
Cigarette filters constructed using only crimped cellulose acetate tow lack activity in reducing smoke gas phase constituents such as formaldehyde, acetaldehyde, acrolein, 1,3-butadiene and benzene. Adsorbing materials such as activated carbons, zeolites, silica gels and APS silica gels capable of removing gaseous constituents from cigarette smoke may be deposited between the filaments of a cellulose acetate tow during the plug making process. However, the plasticizers (such as triacetin) often used in the process tend to reduce the activity of the included adsorbents. Other methods to include adsorbent materials in cigarette filters include sandwiching granules between cellulose acetate plugs in plug-space-plug configurations. To avoid high resistance-to-draw (RTD), only larger granules are used.
U.S. Pat. No. 6,257,242 discloses a filter element to reduce or eliminate vapor phase components of air or smoke. A first filter section contains activated carbon cloth while a second filter section contains a mixture of catalytic activated carbon and coconut activated carbon. Woven and nonwoven carbon cloth includes fibers transverse to the directional flow of mainstream smoke, and therefore result in less efficient use of carbon for adsorption purposes.